Building emergency lighting for use in the event of power failure is a requirement for public buildings, for example department stores, offices, hospitals, museums, libraries, sporting facilities and the like. Such lighting is required to operate for an extended period by power from internal storage batteries that are put into use when the mains power supply fails. Despite their hunger for power and limited life, such lighting devices have up to now relied on incandescent lamps or fluorescent tubes. Light-emitting diodes (LED's) have low power consumption and long life, but suggestions for their use for building lighting have been few, and the problem of emergency lighting of pathways and areas as opposed to individual signs has not been considered.
Battery powered LED arrays built into building exit signs are disclosed in U.S. Pat. No. 5,303,124. Although the LEDs can illuminate the exit sign, in the disclosed signs they do not have the capacity to provide emergency illumination over large areas of the building. Patents U.S. Pat. Nos. 5,575,459 and 5,463,280 disclose LED lamps for use in exit signs and based on linear arrays of LEDs.
There have been a variety of disclosures of LED-based lamps outside the emergency lighting field. LED-based light strings for Christmas trees are disclosed in U.S. Pat. Nos. 4,984,999 and 5,495,147. LED arrays for vehicle lamps are disclosed in U.S. Pat. Nos. 4,929,866 and 4,935,665. A lamp using LEDs and a concave reflector to provide a concentrated beam is disclosed in U.S. Pat. No. 5,782,553, but such proposals are not appropriate for building emergency lighting where the requirement is to provide large areas of evenly distributed intensity. U.S. Pat. No. 5,838,247 discloses arrays of LEDs to provide the circular lamps for traffic lights. U.S. Pat. No. 5,899,557 discloses LEDs feeding their light into a curved cylindrical lens for generating a controlled beam pattern which is elongated in a defined plane, e.g. for a marine light fitted to a buoy or marker. Light strings based on LEDs are disclosed in U.S. Pat. No. 6,072,280.